Avalanche Advisory February 4, 2011

Good morning! This is Dudley Improta with the West Central Montana Avalanche Center’s avalanche advisory for February 4, 2011.
On all slopes above 5000 feet and steeper than 35 degrees there is a MODERATE avalanche hazard. During MODERATE avalanche hazard there is heightened avalanche hazard on specific terrain features (described in our analysis); you should evaluate snow and terrain carefully. On remaining terrain within the advisory area the avalanche hazard is LOW.

Weather and Snowpack Analysis

After the welcomed 20cm (8 inches) or so of powder snow that fell in the Northern part of the region last Sunday we haven’t received much moisture and the temperatures dropped significantly. This allowed the snow on the surface to remain cold and loose in most locations. Since Wednesday, winds picked up considerably at elevation and on the ridge tops. The cold, loose snow was easily moved by the wind and deposited on lee aspects. The very cold 20cm of snow was sluffing easily on steep slopes all week; both naturally and with human-triggers.  

Our MODERATE avalanche hazard refers to a couple of situations. First, wind slabs have formed on the lee sides of steep open slopes, ridge tops, gullies and couloirs. Observers reported strong, sensitive wind slabs cracking and moving in the top of the Wisherd Ridge bowls yesterday. Secondly, slopes steeper than 35 degrees, when being skied or ridden, may produce sluffs big enough to knock you down or drive you into hazards.

We have this cold, loose snow on the surface right now and surface hoar in some parts of the forecast area. (Surface hoar was noted near Lookout Pass and Lolo Pass. Although surface hoar was reported in the Rattlesnakes this week; it appears, from our tour yesterday, that the wind has destroyed it.) We are concerned with the warming temperatures and snowfall predicted for the weekend; anywhere from 17cm (7-10 inches) at Lookout, Lolo, and the Rattlesnakes to 30cm (1 foot) near Lost Trail Pass and in the Seeley area by Saturday. If we get this warmer, denser snow; it will be sitting on top of the current, colder surface. Our video from yesterday shows the current state of the snowpack in the Rattlesnakes and our concern about the weekend weather forecast.  

Weather Forecast and Avalanche Outlook

A moist and warm Westerly flow has developed and should produce light snow this morning. A Northwest flow and another weather system should push in late today and tonight; this system may produce moderate to heavy snow in the mountains and freezing rain in the valleys.

Expect the avalanche hazard to escalate to CONSIDERABLE with new snow loads at warmer temperatures or possibly rain at higher elevations. All steep slopes above 5000 feet will be suspect with a new load and particular consideration should be given to any wind-loaded areas and the colder Northerly aspects.

Steve will issue the next advisory on Monday, February 7.  Ride and ski safe and have a great weekend.

If you get out and see avalanche activity or want to send us quick snow observations please use our public observations form on the home page of missoulaavalanche.org or write us at [email protected].